1. Field of the Invention
The present invention relates to a hybrid type telephony system and in particular, to a hybrid type telephony system capable of providing a single service to both of a conventional type telephone set contained in a time-division switch and a LAN type telephone set contained a LAN.
2. Description of the Related Art
With reference to FIG. 4, explanation will be given on a conventional telephony system including a private branch exchange unit and a LAN type telephone system.
In FIG. 4, a private branch exchange unit 150 is based on a time-division switch 100. The time division switch 100 has a function for switching 64-Kbps time division data. A central control unit 101 has a function for controlling the entire exchange unit 150. A subscriber circuit 102 and a subscriber circuit 103 are interface circuits of conventional type telephone sets having a function of voice interface and a function to control conventional type telephone sets. A trunk circuit 104 has an interface function for interfacing with other exchange units and apparatuses. A gateway 107 has a function for data conversion between time division data handled by a private branch exchange unit and voice packets handled by a LAN type telephone system. Conventional type telephone sets 105 and 106 may be ordinary analog telephone sets or multi-function telephone sets for a private branch exchange unit. Moreover, a LAN switch 108 has a function for switching packets on the LAN. A LAN 109 and a LAN 110 are LAN networks. A LAN type telephone set 111, a LAN type telephone set 112, and a LAN type telephone set 113 are LAN type telephone sets connected contained in the LAN. A server 114 has a function for a call connection between the LAN type telephone sets 111, 112, 113 and the gateway 107 and other service.
When constructing a telephone service system by using a private branch exchange unit and a LAN type telephone system, the exchange unit 150 is connected to the LAN type telephone system via the gateway 107. In this case, a line having a 64-Kbps speed is used for connection between the trunk circuit 104 of the private branch exchange unit 150 and the gateway 107 while a LAN is used for connection from the gateway 107 to the LAN switch 108.
For example, when calling from the conventional type telephone set 105 to the LAN type telephone set 111, upon off-hook of a receiver of the conventional type telephone set 105, the subscriber circuit 102 connected to the conventional type telephone set detects the off-hook and reports it to the central control unit 101.
The central control unit 101 performs a call processing such as sending a dial tone to the conventional type telephone set 105 and receives a selection signal as a telephone number of the destination from the conventional type telephone set 105. In this case, the selection signal is a combination of an access number for catching the trunk 104 and a telephone number of the LAN type telephone set 111 on the LAN type telephone system. For example, when it is assumed that the access number for catching the trunk 104 is “8” and the telephone number of the LAN type telephone set 111 is “111”, the selection signal will be “8111”.
The central control unit 101, upon recognition of that the selection signal from the conventional type telephone set 105 is “8111”, establishes a communication path between the subscriber circuit 102 and the trunk circuit 104 through the time division switch 100 and transmits a selection signal “111” from the trunk 104 to the gateway 107.
With this, the gateway 107 recognizes that the call from the trunk 10 is a call to the LAN type telephone set 111 and performs a call processing to the server 114 including inquire of the IP address of the LAN type telephone set 111, so that a voice information packet is transmitted and received between the gateway 107 and the LAN type telephone set 111. This enables communication between the conventional type telephone set 105 and the LAN type telephone set 111.
Moreover, Japanese Patent Publication 10-308777 discloses a voice exchange system of a LAN terminal in which a LAN subscriber circuit is contained in a line interface block of an electronic exchange unit to which a terminal such as a LAN type telephone set is connected via a LAN. In this voice exchange system, voice data from a LAN terminal always passes through the LAN subscriber circuit and the main switch of the electronic exchange unit. Accordingly, even a communication between two LAN type telephone sets on the LAN is performed via a path formed by a first LAN type telephone set, a first LAN subscriber circuit, the main switch, a second LAN subscriber circuit, and a second LAN type telephone. In this case, voice compression is performed as follows. Firstly, voice data is compressed in the first LAN type telephone set 1 and transmitted via the LAN to the first LAN subscriber circuit, where the voice data is restored. After this, the voice data is transmitted from the first LAN subscriber circuit to the main switch and then compressed in the second LAN subscriber circuit. The compressed voice data is restored in the second LAN type telephone set.
In the conventional telephony system using a private branch exchange unit and a LAN type telephone system, the private branch exchange unit and the LAN type telephone system operate as completely different systems which are connected via a gateway. Accordingly, it is impossible to provide a unitary service. For example, for a communication between a conventional type telephone set contained in a private branch exchange unit and a LAN type telephone set it is necessary to input an access number (such as 8) indicating a route via a gateway and a telephone number (such as 111) in a destination system.
Moreover, in the aforementioned voice exchange system, voice data is significantly deteriorated because compression-restore is repeated twice. Moreover, compression and restore require not little time. Furthermore, when transmitting a voice packet using a LAN, the voice packet transmission is delayed with a delay time fluctuating depending on the packet. To cope with this, a plurality of voice packets is accumulated for a predetermined period of time before restoring the voice packets, so as to absorb the fluctuation. However, while repeating the compression-restore process, the time for absorbing the fluctuations becomes great resulting in a great delay time, greatly affecting the voice quality.